Method for fabricating a laser diode using a reflective layer including an air layer

ABSTRACT

A laser diode that uses air as a reflective layer, thereby enhancing reflectance with respect to an oscillating laser beam, and a method for fabricating such a laser diode are provided. The laser diode includes a substrate, a laser oscillating layer formed on the substrate, an upper electrode formed on the laser oscillating layer, and a reflective layer formed at one side of the laser oscillating layer, wherein the reflective layer comprises air layers. According to the laser diode and the method for fabricating the same, it is possible to form a reflective layer having a higher reflectivity with a reduced number of pairs of reflective layers, thereby making a laser diode whose threshold voltage is reduced and which can produce a high-output laser beam.

This application is a divisional of U.S. patent application Ser. No.10/152,037, filed May 22, 2002, now U.S. Pat. No. 6,603,212, whichclaims priority to Patent Application Number 2001-28089 filed in Rep. ofKorea on May 22, 2001, herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser diode and a method forfabricating the same, and more particularly, to a laser diode that usesair as a reflective layer, thereby enhancing reflectance with respect toan oscillating laser beam, and a method for fabricating such a laserdiode.

2. Description of the Related Art

In general, a semiconductor laser diode is a structure in which a bufferlayer, a laser oscillating layer that is composed of a buffer layer, alower clad layer, an active layer and an upper clad layer, and a caplayer and an upper electrode are sequentially stacked. Also, a lowerelectrode is formed below the semiconductor substrate.

When a predetermined voltage is applied across the upper electrode andthe lower electrode, an electromagnetic wave of predeterminedwavelength, which is generated in the active layer and oscillatesbetween the both sides of the laser oscillating layer, is emitted out ofa side of the laser oscillating layer. This is an edge emitting laserdiode. Laser diodes have various uses, but have been mainly used inoptical pickups for writing and reading data on a writing medium such asdigital versatile discs (DVDs).

It is required that a mirror layer with a high reflectance be attachedto a side of the laser oscillating layer so as to generate a high-outputlaser beam. The mirror layer is obtained by coating a thin layer thathas a high reflectance on one side of the laser oscillating layer.Today, there is a lot of ongoing research into the formation of such athin layer.

In the past, as can be seen in FIG. 1, a reflective layer 15 wasobtained by alternatively depositing first and second dielectric layers13 and 14 which have different refractive indexes several times on aside of a laser oscillating layer 12 from which laser light is emitted,thereby making a laser diode whose threshold voltage is reduced andwhich can produce a high-output laser beam. Here, the thickness of eachof the first and second dielectric layers 13 and 14 is calculated byλ/(4n), wherein λ denotes the wavelength of laser beam emitted from thelaser oscillating layer 12, and n denotes the refractive index of eachdielectric layer with respect to the wavelength of laser beam emitted.In general, the greater the difference between the refractive indexes oftwo dielectric layers, and the greater the number of layers depositedare, the greater the reflectance is. Here, the dielectric layers 13 and14 are alternately deposited several times to form several pairs of thedielectric layers 13 and 14.

It is possible to select various dielectric materials for the dielectriclayers 13 and 14 which make up the reflective layer 15, according to thewavelength of laser beam emitted from the laser oscillating layer 12.Preferably, the dielectric layers are formed of SiO₂ and TiO₂ becausethe difference between their refractive indexes is greater than any twoother dielectric materials. Once dielectric materials are selected, thefirst dielectric layer 13 is formed, and then second dielectric layer 14whose refractive index is larger than that of the first dielectric layer13 is formed. If these dielectric layers 13 and 14 are formed of SiO₂and TiO₂, the difference between their refractive indexes is about 1.35with respect to a laser wavelength of 410 nm. To give the reflectivelayer 15 a high reflectance, e.g., 90% or more, it must be made of atleast three pairs of the first and second dielectric layers 13 and 14.Therefore, for high reflectance, the number of pairs of the dielectriclayers 13 and 14 must be greater than a predetermined number. However,as the number of pairs of the dielectric layers 13 and 14 increases, ittakes more time to form and etch them to make a thin layer.

Hereinafter, a conventional reflective layer and its manufacturingprocess will be described with reference to FIGS. 2A through 2C.Referring to FIG. 2A, a laser oscillating layer 22 from which a laserbeam is emitted is deposited on a semiconductor substrate 21, and then,two dielectric materials are alternatively deposited to form dielectriclayers 23 and 24 that together make up a reflective layer 25. When thedielectric layers 23 and 24 are formed, a dielectric material whoserefractive index is comparatively low is first deposited, and then adielectric material whose refractive index is relatively high is laterdeposited. The dielectric layers 23 and 24 are alternately depositedseveral times to form a desired number of pairs thereof.

As described above, in order to obtain a reflectance of 90%, at leastthree pairs of the dielectric layers 23 and 24 must be deposited, evenif they are formed of SiO₂ and TiO₂ which have a greater difference inrefractive index than any two other materials, as shown in FIG. 2B.During this process, several pairs of the dielectric layers 23 and 24are deposited on a portion of the semiconductor substrate 21 as well asa side of the laser oscillating layer 22 from which a laser beam isemitted. Thereafter, a portion of the several pairs of the dielectriclayers 23 and 24 formed on an upper electrode 26 must be removed toconnect the upper electrode 26 with the outer electrode as shown in FIG.2C. This is because the dielectric layers 23 and 24 are non-conductivelayers that block the flow of current. Accordingly, a conventional laserdiode is disadvantageous in that it takes a lot of time to deposit andetch the dielectric layers 23 and 24, especially, when there are manypairs of the dielectric layers 23 and 24.

SUMMARY OF THE INVENTION

To solve the above problem, it is a first object of the presentinvention to provide a laser diode in which the number of pairs ofdielectric layers is reduced due to an air layer formed on a dielectriclayer with a low refractive index, and a reflective layer of higherreflectance is formed, thereby giving the laser diode a reducedthreshold voltage and a high output.

It is a second object of the present invention to provide a method forfabricating such a laser diode.

To achieve the first object, there is provided a laser diode including asubstrate, a laser oscillating layer formed on the substrate, an upperelectrode formed on the laser oscillating layer, and a reflective layerformed at one side of the laser oscillating layer, wherein thereflective layer comprises air layers.

Preferably, the reflective layer has a structure in which an air layerand a dielectric layer are alternately deposited several times and ismade of at least two pairs of an air layer and a dielectric layer.

Preferably, the dielectric layers include a TiO₂ layer.

To achieve the second object, there is provided a method of fabricatinga laser diode including the steps of: (a) forming a laser oscillatinglayer on a substrate, and then, forming an upper electrode on the laseroscillating layer; (b) forming sacrificial layers and dielectric layersalternately at one side of the laser oscillating layer; and (c) etchingthe sacrificial layers selectively.

Preferably, during step (b) the sacrificial layers and the dielectriclayers are alternately deposited to form two pairs of these layers.

Preferably, during step (b), the sacrificial layers and the dielectriclayers are formed by sputtering.

Preferably, this method further includes removing the sacrificial layersand the dielectric layers from the upper electrode after the step (b),and the sacrificial layers and the dielectric layers are removed fromthe upper electrode by RIE.

Preferably, during step (c), the sacrificial layers are selectivelyetched by wet etching, using a buffered oxide etchant (BOE).

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore apparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a cross-sectional view of a conventional laser diode;

FIGS. 2A through 2C are cross-sectional views for explaining a methodfor fabricating a reflective layer of a conventional laser diode;

FIG. 3 is a cross-sectional view of a laser diode according to thepresent invention;

FIGS. 4A through 4D are views for explaining a method for fabricating areflective layer of a laser diode according to the present invention;

FIGS. 5A and 5B are photographs of a reflective layer of a laser diodeaccording to the present invention, taken by a scanning electronmicroscope (SEM). In detail, FIG. 5A is an SEM photograph of a laserdiode in which a sacrificial SiO₂ layer and a dielectric TiO₂ layer areformed on a semiconductor substrate and a side of a laser oscillatinglayer. FIG. 5B is an SEM photograph of the laser diode of FIG. 5A inwhich the sacrificial layer is selectively etched; and

FIG. 6 is a graph showing the reflectance of a reflective layer withrespect to the wavelength of a laser beam emitted from a laseroscillating layer in a laser diode according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the structure of a laser diode that uses a reflective layerincluding an air layer will now be explained with reference to FIG. 3.Referring to FIG. 3, a laser oscillating layer 32 is formed on asemiconductor substrate 31. A reflective layer 35, which is made byalternatively depositing an air layer 33 and a dielectric layer 34several times, is formed on a side of the laser oscillating layer 32.The laser oscillating layer 32 has a structure in which a lower cladlayer, an active layer and an upper clad layer are sequentiallydeposited. Here, the reflective layer 35 can be also applied to an edgeemitting laser diode.

In the laser diode, a laser beam of a predetermined wavelength, which isgenerated in a laser oscillating layer, causes constructive interferencewhile oscillating between both sides of a laser oscillating layer. As aresult, the laser beam has predetermined energy, and then is emitted outof a side of the laser oscillating layer. The reflective surface of oneside of the laser oscillating layer and the opposite side from which thelaser beam is emitted, is formed to have a reflectance of 90% or morewith respect to the laser beam.

The reflective layer with this reflective surface is made by depositingseveral pairs of dielectric layers of two different materials. At thistime, preferably, the difference between the refractive indexes of thetwo different dielectric materials is larger and the reflective layer iscomposed of several pairs of the dielectric layers.

The reflective layer of a conventional laser diode is usually formed ofdielectric materials such as SiO₂, TiO₂, Al₂O₃, ZrO₂ and HfO₂. Two ofthese dielectric materials are selected in consideration of theirrefractive indexes with respect to the laser beam emitted, and then arealternatively deposited several times to form the reflective layer.

On the other hand, in a laser diode according to the present invention,the reflective layer 35 is formed of one dielectric material and airrather than two different dielectric materials. That is, several pairsof an air layer 33 and a dielectric layer 34 form the reflective layer35 together. Here, the refractive index of air is 1, and therefore islower than any other dielectric material.

Referring to FIGS. 4A through 4D, a method for fabricating a laser diodethat uses a reflective layer including an air layer, according to thepresent invention, will now be described in detail. First, as shown inFIG. 4A, a laser oscillating layer 42, which is made of a lower cladlayer, an active layer and an upper clad layer, is formed on a substrate41. Then, an upper electrode 47 is deposited on the laser oscillatinglayer 42.

Next, as shown in FIG. 4B, a sacrificial layer 43 is formed of amaterial that can be selectively etched by sputtering, on the resultantstructure of the substrate 41. Thereafter, a dielectric material with ahigh refractive index is deposited to form a dielectric layer 44 on thesacrificial layer 43. Then, the sacrificial layer 43 and the dielectriclayer 44 are alternately formed several times to form several pairs.Here, the points on which the sacrificial layers 43 are deposited arethe places where air layers 45 are to be formed during the subsequentprocess. The thicknesses of the sacrificial layer 43 and the dielectriclayer 44 are calculated by the formula λ/(4n). Here, λ denotes thewavelength of the laser beam emitted from the laser oscillating layer42, and n denotes the refractive index of air and the dielectric layer44 with respect to the wavelength of the laser beam emitted. Since therefractive index of air is 1, the thickness of each point where the airlayers 45 is calculated by λ/4.

Here, the number of pairs of the sacrificial layer 43 and the dielectriclayer 44 varies according to the refractive index of the dielectricmaterial of the dielectric layer 44. For instance, if the dielectriclayer 44 is formed of TiO₂, the difference between its refractive indexand the refractive index of air is 1.83 with respect to a laser beamwavelength of 410 nm. Therefore, two pairs of the sacrificial layer 43and the dielectric layer 44 are sufficient to obtain a reflectance ofmore than 95% as shown in FIG. 4B.

Then, predetermined portions of the sacrificial layers 43 and thedielectric layers 44, which are non-conductive layers, are removed toconnect the upper electrode 47 to an outside power source. Preferably,in the case of an edge emitting laser diode according to the presentinvention, a reflective layer 46 that is composed of pairs of thedielectric layer 44 and the air layer 45 is formed at one side of thelaser oscillating layer 42. However, thin layers such as the sacrificiallayers 43 and the dielectric layers 44 may be formed on the upperelectrode 47 during the deposition of these layers. Preferably, thesethin layers are removed by reactive ion etching (RIE).

Next, the sacrificial layers 43 are selectively removed from one side ofeach of the laser oscillating layer 42 by wet etching. As a result, theportions from which the sacrificial layers 43 are removed are filledwith air as shown in FIG. 4D, thereby being the air layers 45. Thus, atone side of each of the laser oscillating layer 42 and the upperelectrode 47, the air layers 45 and the dielectric layers 44 arealternately deposited several times to form several pairs, thus beingthe reflective layer 46. Even if the sacrificial layers 43 are notcompletely removed, i.e., remnant sacrificial layers 43′ are present, alaser diode according to the present invention can operate normally ifthe air layers 45, rather than the sacrificial layers 43, are formed atone side of the laser oscillating layer 42. However, the remnantsacrificial layers 43′ must not be in the line of the laser beam emittedfrom the laser oscillating layer 42.

FIGS. 5A and 5B are photographs of a reflective layer of a laser diodeaccording to the present invention, taken by a scanning electronmicroscope (SEM). In detail, FIG. 5A is an SEM photograph of the laserdiode having sacrificial layers (SiO₂) and dielectric layers (TiO₂) atone side of a substrate and a laser oscillating layer, and FIG. 5B is anSEM photograph of the laser diode in which the sacrificial layers areremoved by selective etching.

From FIG. 5B, it is noted that the reflective layer has a structure inwhich an air layer, a dielectric layer, an air layer, and a dielectriclayer are sequentially deposited at one side of the substrate and thelaser oscillating layer as shown in FIG. 4D.

FIG. 6 is a graph showing the reflectance of the wavelength of the laserbeam. emitted from a laser diode, according to the present invention,including a reflective layer that has air layers. Here, the reflectivelayer is formed by alternately depositing an air layer and a TiO₂ layertwo times, i.e., two pairs.

For instance, an SiO₂ layer and a TiO₂ layer are sequentially formed.Then, an SiO₂ layer and a TiO₂ layer are sequentially formed once moreto form two pairs of the SiO₂ layers and the TiO₂ layers. Here, withrespect to a laser beam wavelength of 410 nm, the SiO₂ layer is formedto a thickness of 103 nm in consideration of the refractive index ofair, i.e., 1, because it will be selectively etched during thesubsequent process, and then, the TiO₂ layer is formed to a thickness of37 nm because its refractive index is 2.83.

After removal of the reflective layer from the upper electrode, theremnant reflective layer, which is formed at one side of the laseroscillating layer, may be selectively etched using a buffered oxideetchant, e.g., diluted hydro fluorine acid, to remove the SiO₂ layers.Thus, the resulting reflective layer has a structure in which an airlayer, a TiO₂ layer, an air layer and a TiO₂ layer are sequentiallystacked. Then, with respect to various laser beam wavelengths, thereflectance of the reflective layer is measured, and the results showthat the reflective layer has a reflectance of more than 90% for thewavelengths below 550 nm. in particular, the reflectance of thereflective layer is 97% at the wavelength of 410 nm. That is, thereflective layer has higher reflectance for a wider range of wavelengthsthan conventional reflective layers, and further, a laser diodeaccording to the present invention can be stable irrespective of awavelength variation, due to the reflective layer.

While the present invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

In a reflective layer of a laser diode according to the presentinvention, the one of two different dielectric layers used inconventional laser diodes that has the lower refractive index isreplaced with an air layer. Accordingly, it is possible to form areflective layer having a higher reflectance with a reduced number ofpairs of dielectric layers and air layers, thereby making a laser diodewhose threshold voltage is reduced and which can produce a high-outputlaser beam.

What is claimed is:
 1. A method of fabricating a laser diode, comprisingthe steps of: (a) forming a laser oscillating layer on a substrate, andthen, forming an upper electrode on the laser oscillating layer; (b)forming sacrificial layers and dielectric layers alternately at one sideof the laser oscillating layer; and (c) removing the sacrificial layersselectively, wherein the removing step provides air layers in place ofthe sacrificial layers, the air layers being arranged next to thedielectric layers in an alternating manner.
 2. The method of claim 1,wherein during step (b) the sacrificial layers and the dielectric layersare alternately deposited to form two pairs of these layers.
 3. Themethod of claim 1, wherein during step (b), the sacrificial layers andthe dielectric layers are formed by sputtering.
 4. The method of claim1, further comprising removing portions of the sacrificial layers andthe dielectric layers from the upper electrode after the step (b). 5.The method of claim 4, wherein said portions of the sacrificial layersand the dielectric layers are removed from the upper electrode by RIE.6. The method of claim 1, wherein during step (c), the sacrificiallayers are selectively etched by wet etching, using a buffered oxideetchant (BOE).
 7. The method of claim 1, wherein the removing stepincludes the step of etching the sacrificial layers selectively.
 8. Themethod of claim 1, wherein the removing step provides a reflective layercomprising the air layers.